Image forming method, screen set and image forming apparatus

Abstract

An image forming apparatus for forming a color image of CMK color plates by electrophotography system in which a K color plate having highest visibility among CMYK colors is composed of a rational number mesh. Profile of halftone dots ( 201 Kh) becomes constant substantially and the factor for generating streak and unevenness is suppressed. Furthermore, C color plate and M color plate are formed of super cells and the K color plate, C color plate and M color plate are formed at an interval of 30°. Consequently, a Rosetta formed of halftone dots ( 201 Ch, 201 Mh, 201 Kh) of CMK is optimized and the factor for generating moiré component is suppressed.

1. An image forming method for forming a color image with at least C, M, K plates according to an electrophotographic process, comprising the steps of:
 forming a K plate as a rational screen, wherein assuming a screen angle θ is expressed as θ=arctan(m/n) where m and n are natural numbers, then a size of a threshold matrix satisfies a positive integral multiple of m 2 +n 2 ; 
 forming C and M plates as supercells; and 
 spacing respective screen angles of the K plate, the C plate, and the M plate at respective intervals of 30°. 
 
   
   
     2. An image forming method according to  claim 1 , wherein said screen angle of said K plate comprises an angle other than 0° or 45°. 
   
   
     3. An image forming method according to  claim 1 , wherein a value produced by dividing an output resolution by a ruling for each of said C, M, K plates is smaller than 12. 
   
   
     4. An image forming method according to  claim 1 , wherein said size of the threshold matrix of said K plate is within a range of from 80×80 to 256×256. 
   
   
     5. An image forming method according to  claim 1 , wherein a Y plate is added to form color images using the C, M, Y, K plates, said Y plate being formed as the same rational screen as said K plate, while being shifted in phase from said K plate. 
   
   
     6. An image forming method according to  claim 5 , wherein said Y plate is shifted in phase from said K plate within a range of from 150° to 210°. 
   
   
     7. An image forming method according to  claim 1 , wherein variations of rulings for the respective colors are set to 10 LPI or less. 
   
   
     8. An image forming method according to  claim 1 , wherein said respective screen angles of said K plate, said C plate and said M plate are spaced at respective intervals of 30°±1.5°. 
   
   
     9. A screen set for use in forming a color image with at least C, M, K plates according to an electrophotographic process, comprising:
 a K plate formed as a rational screen, wherein assuming a screen angle θ is expressed as θ=arctan(m/n) where m and n are natural numbers, then the size of a threshold matrix satisfies a positive integral multiple of m 2 +n 2 ; and 
 C and M plates formed as supercells, 
 wherein respective screen angles of said K plate, said C plate and said M plate are spaced at respective intervals of 30°. 
 
   
   
     10. An image forming apparatus for forming a color image with at least C, M, K plates according to an electrophotographic process, comprising:
 a K plate formed as a rational screen, wherein assuming a screen angle θ is expressed as θ=arctan(m/n) where m and n are natural numbers, then the size of a threshold matrix satisfies a positive integral multiple of m 2 +n 2 ; and 
 C and M plates formed as supercells, 
 wherein respective screen angles of said K plate, said C plate and said M plate are spaced at respective intervals of 30°.